If a large company is not updating their site very often, then you do not have a great value proposition anyway. Your core product is ease of updating/editing and customization, no? Makes sense to charge based on that aspect.

Of course you need some sort of traffic restriction for things not to spiral out of control. Putting this restriction on actual MB's transferred is an acceptable safeguard. Cache bandwidth is cheap using the right strategies, so your bounds could be pretty high while keeping it profitable.